High-frequency variable power output system



Feb. 6, 1951 T. u. FOLEY ET AL 2,540,613

HIGH-FREQUENCY VARIABLE. POWER OUTPUT SYSTEM Fil ed April 8, 1947 2Sheets-Sheet 1 T0 F/LAMENT HEAT/N6 SUPPLY INVENTORS THOMAS U. FOLEYGEORGE WJKLINGAMAN ATTORN EY Feb. 6, 1951 T. u. FOLEY ETAL 2,540,613

HIGH-FREQUENCY VARIABLE POWER OUTPUT SYSTEM Filed April 8, 1947 2Sheets-Sheet 2 INVENTORS THOMAS u. FOLEY BY GEORGE w. KUNGAMAN ATTORNEYPatented Feb. 6, 195i HIGH-FREQUENCY VARIABLE POWER OUTPUT SYSTEM ThomasU. Foley, Magnolia, N. J andGeorge W.

Kiingaman, Lynnport, Pa., assignors to Radio Corporation of America, acorporation of Dela= ware Application April 8, 1947, serial N; "140,148Claims. (01. 25o --36) This invention relates to systems utilizingvariable power outputs, such as high frequency dielectric heatingsystems, and particularly to a method of and apparatus for efficientlyenabling the output of such a system to be varied over a very wide rangein a continual and gradual manner.

An object of the invention is to provide a high frequency dieectricheating system whose power output obtained from the tank circuit can bevaried anywhere in the range of 400 watts to kilowatts gradually andcontinuously, without introducing arc-over complications.

Another object of the invention is to provide a capacity couplingarangement between the output tank circuit of an electron dischargedevice system and a load, by means of which the output power availableto the load can be continuously and gradually varied over an extremelywiderange,

A feature of the invention lies in the use of a capacity potentiometercomprising, in eifect, three physically parallel plates, the outer twoplates of which are stationary and the center plate of which iscoupledto the load and movable in a direction at right angles to theouter plates.

Other objects and features of the invention will appear from a readingof the following description, in conjunction with a drawing, wherein:

Fig. 1 illustrates an embodiment of the invention used in a highfrequency dielectric heating system; and

Figs. 2 and 3 are different views of the details of the variablecapacity coupling feature of the invention. I

Throughout the figures of the drawing, the same parts are indicated bythe same reference numerals.

In Fig. 1 there is shown a high frequency dielectric heating systemcomprising a vacuum" through a parasitic suppressor circuit l3 to theother terminal D of the tank inductance l2, The filament is connected toground and through a grid leak resistor M to a point C of zero radiofrequency potential on the tank inductance I2.

A grid excitation capacitor [5 is connected besuch that'there isobtained a Vernier adjustment,- in the operation of the variablecapacitor I5 Heating current for the filament is supplied throughiron-core transformer T, polarizing potential from the positive terminal13-}- of a source of unidirectional supply is fed to the anode A througha radio frequency choke coil IT. A radio frequency bypass condenser ll}is connected between the B+ supply lead and;

with the capacitor portion of the tank circuit comprising the gridexcitation capacitor l5 in series with the effective anode to groundcapacities represented by the dash line capacitor 20,

Output energy from the tank circuitis taken from terminal B on the tankinductor 12. This terminal B is at a point of low radio frequencypotential and low impedance in order to facilitate loading, reduceharmonics and alsoreduce; reaction of the load on the tank; Thisterminal B is coupled to one outer plate of a three-plate. motor drivencapacity potentiometer 2|. The; capacity potentiometer 2| is independentof the oscillator per se, and is not depended upon to cause oscillationsto occur in the tank circuit. This potentiometer 21 comprises threemetallic. platesQP, PI and P2 whichare physically parallel to oneanother. The outer plates P and P2:. are respectively connected to thetank inductor l2 and to ground. The central plate PI is con nectedthrough a transmission line TL to the load. In effect, this load corn-t.1 prises a coil 23 which tunes with the efiective load. capacit acrossthe electrodes 24. Th dielectric or non-metallic batch of material to beheated is.-. adapted to be placed between the plates 24 in the orapplicator unit 22.

applicator unit 22.

In using the dielectric unit circuit of Fig. 1; it.

will be evident that different materials t be heated between theelectrodes 24 in the load will require different output power from theoscillator. It is therefore necessary to provide an arrangement forenabling the output from the Colpitts oscillator to vary over a widerange. The capacity potentiometer 2| of the invention achieves thisdesired result in a gradual and continual manner over a very wide poweroutput range.

In effect, the plates P, PI and P2 have relative- 1y wide surface areas.The outer plates P and P2 are stationary, while the center plate PI ismovable in an axial direction toward or away from P2. When the plate PIis moved toward P2 and away from plate P, there will be a reduction inthe power output to the load. Conversely, when the plate Pl moves awayfrom the ground plate P2 and toward the plate P which is connected tothe tank inductor, the power output to the load will increase.

In one embodiment of the invention, the circuit of Fig. 1 was used toproduce kilowatts maximum power output at megacycles. The tank inductorI2 was in the form of a U-shaped fiat plate linear stub arrangement witha shorting bar to adjust the frequency to certain specifled values.Thus, with no shunt capacity directly connected across the terminals Eand D of the tank inductor [2, it was possible to obtain power output ata frequency of the order of 40 mega.- cycles, whereas with the additionof certain values of capacities across the terminals E and D of the tankinductor, it was possible to obtain power output at frequencies of 13megacycles and 27 megacycles depending upon the values of the addedcapacitors.

of the order of 13 megacycles and 2'7 megacycles, the tank inductor waschanged and shunted with suitable capacities, and the values of theanode choke coil I1 and the arasitic suppressor l3 were also changed tosuitable values. The oscillator was able to suppl full power at any oneof these three frequencies, 13 megacycles, 27 megacycles and 40megacycles.

The voltage developed across the terminals E and D in the embodimentactually tried out in practice, was about 6700 volts R. M. S. The voltage between the terminals E and C was 5430 volts R. M. S. The voltagebetween terminals B and C was 1800 volts R. M. 8., while the voltagebetween terminals C and D was 1270 volts R. M. S.

Suitable cooling fluid for cooling vacuum tube was supplied by a blowercircuit. Although aircooling was actually employed, it will be evidentthat a water tube can be employed. The vacuum tube actually was an RCA889R-A power triode which was employed in parallel with another similarpower triode, in order to obtain greater power output than can besupplied with one such tube alone.

The details of the capacit potentiometer 2| are shown more clearly inFigs. 2 and 3. The outer plate P and the middle plate Pl had dimensionsof the order of 12 inches by 20 inches, while the other outer plate P2formed part of the metallic housing for th apparatus. The plate P wassupported at both ends through insulators I mounted on supports, S, inturn, fastened to the metallic grounded housing plate P2. The housingplate P2 is provided with an aperture at its center across which isbridged a metal plate P4 attached to P2. Attachedto plate P4 is a rackand gear mechanism G which is linked to a reversible motor M. The rackand gear-mechanism is linked to the central plate Pl through a shaft SI,in turn, secured to a plate P5 mounted on three insulators 52.

The motor M is controlled by power supplied to connections 60. Thismotor can revolve in either of two directions depending upon theoperation of either of a pair of switches. switches are preferablypush-buttons. control could also be accomplished with an automaticsensing device.)

When the oscillator was usedto produce output power at a radiofrequency.

These (Motor In the operation of the system of Fig. 1, the parasiticfrequencies have been largely suppressed by the use of the parasiticgrid suppressor l3 and by the elimination of inductance in the gridreturn. Harmonics were reduced to a low value by tapping the output feedlin to the terminal B which is close to the zero potential point on thetank inductor I2, and by the use of high-Q tank inductance. Thecapacities used for the grid excitation control, for anode blocking, forfilament bypassing purposes, and for stabilizing the tank were of thelow self-inductance and high current-carrying capacity kind. The tankstabilizing capacitors, not shown, which were used across the tankinductor [2 for obtaining a change in frequency to 13 megacycles or 27megacycles, were of the vacuum type.

Suitable interlocking, control and rectifier circuits of theconventional type were employed, and the entire system was metallicallyenclosed. Low pass radio frequenc filters were used to prevent powerline radiation.

By means of the capacit potentiometer of the arrangement, it has beenpossible to obtain a power output variation from 15 kilowatts maxi mumdown to 300 to 400 watts in a gradual and continuous manner, and withoutintroducing arcing complications. this range of output power by means ofthe capacity potentiometer is merely given by way of example only, andthat other ranges of power output variation may be obtained by the useof our particular type of capacity potentiometer.

The term ground used in the appended claims is not limited to an actualearthed connection, and is deemed to include the chassis for theapparatus.

What is claimed is:

1. In a dielectric heating system, an electron discharg deviceoscillation generator having an oscillatory output circuit, a load to beheated, and a coupling circuit between said load and output circuit,said coupling circuit forming no integral part of said oscillatorycircuit and including a capacitor having three spaced electrodes one ofsaid electrodes being movable relative to thecircuit to said load, saidcapacitor arrangement.

being independent of said tank circuit and comprising three physicallspaced and parallel metallic plates, a connection from one outer platetosaid tank circuit, a connection from said other outer plate to saidcathode, a connection from the middle plate to said load, and meansfor.v moving the middle plate in a direction toward or awa from one ofsaid outer plates while maintaining parallelism with both of said outerplates.

3. In a dielectric heating system, an electron discharge deviceoscillator having a cathode and a tank circuit, a load to be heated, anda capacitor arrangement coupling said tank circuit to said load, saidcapacitor arrangement being independent of said tank circuit andcomprising.

three physically spaced and parallel metallic plates, the outer platesbeing stationary and the middle plate being movable relative to theouter plates, 9. connection from one outer plate to aid It should beunderstood that tank circuit, a connection from said other outer plateto said cathode, a connection from the middle plate to said load, andmeans for moving the middle plate in a direction toward or away from oneof said outer plates while maintaining parallelism with both of saidouter plates.

4. An electron discharge device dielectric heating system capable ofsupplying a Wide range of output power to a load to be heated,comprising a vacuum tube having a tank circuit including a tankinductor, a capacitor arrangement independent of said tank circuit andhaving three physicall parallel metallic plates, the two outer ones ofwhich are stationary, a connection from one outer plate to said tankinductor, a connection from the other outer plate to ground, and aconnection from the middle plate to said load, and means for moving themiddle plate in a gradual manner relative to the outer plates whilemaintaining substantial parallelism between said plates, to thereby varythe amount of output power supplied to said load.

5. A high frequency heating system comprising an electron dischargedevice generator having a tank circuit including a tank inductor and acathode electrode coupled to a point of zero radio frequency potentialon said inductor, a transmission line adapted to be coupled to thematerial to be heated, a capacitor arrangement independent of said tankcircuit and comprising three physically parallel plates, a connectionfrom one outer plate to a point on said inductor relatively close to thepoint of zero radio frequency potential, a connection from the otherouter plate to ground, a connection from the middle plate to saidtransmission line, and means for moving said middle plate relative tosaid outer plates while maintaining parallelism therebetween.

6. A high frequency heating system comprising an electron dischargedevice generator havin a tank circuit including a tank inductor and acath- Ode electrode coupled to a point of zero radio frequency potentialon said inductor, a transmission line adapted to be coupled to thematerial to be heated, a capacitor arrangement independent of said tankcircuit and comprising three physically parallel plates, the outer twoplates of which are stationary, a connection from one outer plate to apoint on said inductor relatively close to the point of zero radiofrequency potential, a connection from the other outer plate to ground,a connection from the middle plate to said transmission line, and meansfor moving said middle plate relative to said outer plates in adirection perpendicular to the planes of said plates while maintainingparallelism therebetween.

7. An electron discharge device heating system capable of supplying awide range of output power to a load adapted to be heated, comprising avacuum tube having a tank circuit including a tank inductor, a capacitorarrangement independent of said tank circuit and having three physicallyparallel plates the two outer ones of which are stationary, a connectionfrom one outer plate to said tank inductor, a connection from th otherouter plate to ground, and a connection from the middle plate to saidload, and a reversible motor for moving the middle plate in a gradualmanner relative to the outer plates in a direction at right angles tosaid plates while maintaining substantial parallelism between saidplates, to thereby vary the amount of output power supplied to saidload.

8. A high frequency dielectric heating system comprising a Colpitts typevacuum tube oscillator having a tube including anode, cathode and gridelectrodes, a tank circuit for said oscillator including a tank inductorof distributed constants, a connection from one end of said tankinductor to said anode, a connection including a parasitic suppressorfrom the other end of said tank inductor to said grid, a connection fromsaid cathode to a point of zero radio frequency potential on said tankinductor, a connection from said cathode to ground, a load adapted to beheated, and a variable capacitor arrangement independent of said tankcircuit and coupling said load to said tank inductor, said capacitorarrangement including three physically parallel metallic plates, theouter two plates being stationary, a connection from one outer plate toa point of relatively low radio frequency potential on said tankinductor, a connection from the other outer plate to ground, and areversible motor linked to the middle plate for moving the sam in adirection at right angles to said plates so as to vary the amount ofoutput energy to said load while maintaining parallelism of said plates.

9. In a high frequency heating system, in combination, a source ofalternating current power, a load to be heated, and a variable capacitorarrangement for varying th amount of power fed from said source to saidload, said capacitor arrangement being independent of said source andcomprising three physicall parallel plates, a connection from one outerplate to said source, a connection from the other outer plate to ground,a connection from the middl plate to said load, means for moving saidmiddle plate in a direction at right angles to the planes of all threeplates while maintaining the physically parallel relationship, and meansfor supporting said outer two plates in fixed spacial relation to eachother.

10. In a high frequency heating system, an electron discharge deviceoscillation generator having a cathode and a tank circuit, a load to beheated, and a capacitor arrangement coupling said tank circuit to saidload, said capacitor arrangement being independent of said tank circuitand comprisin three physically spaced and parallel metallic electrodesone of which is positioned bteween the other two, a connection from anouter electrode to said tank circuit, a connection from the oppositeldisposed electrode to said cathode, a connection from the middleelectrode to said load, and means for moving the middle electroderelative to said other electrodes While maintaining parallelism withboth of said other electrodes.

THOMAS U. FOLEY. GEORGE W. KLINGAMAN.

REFERENCES CKTED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,036,084 Roder Mar. 31, 19362,412,553 Albin Dec, 17, 1946 2,473,188 Albin June 14, 1949 FOREIGNPATENTS Number Country Date 415,464 Great Britain Aug. 27, 1934 OTHERREFERENCES Dielectric Heating Fundaments, by D. Venable, Electronics,Nov. 1945, pages -124.

